The present invention relates to a method and apparatus for thickening lime mud by means of a disc filter. The invention also relates to a method of cleaning the filter sectors of a disc filter. The method and apparatus according to the invention are especially suitable for continuously renewing the precoat layer and/or washing the wire surfaces.
Prior to the lime reburning kiln, the lime mud having a dry solids content of about 25% is thickened usually to a dry solids content of about 70-85% by means of a drum vacuum filter. A so-called precoat filtering layer is formed on the surface of the filter drum by lime mud, the thickness of which is 10-20 mm. The lime mud thickened on the precoat is removed therefrom by means of a scraper, subsequent to which the lime mud falls on a conveyor belt, which then feeds the lime mud to a lime reburning kiln.
The surface layer of the precoat gets clogged while the filtering proceeds and must be removed from time to time. The technique normally used is to move the automatic scraper gradually inwards following a predetermined program, thus removing the clogged surface layer. After the scraper has moved in several steps closer to the drum the rest of the precoat is removed and a new precoat is formed.
All the movements of the scraper described above and the removal of the whole precoat from the surface of the drum cause disturbances in the operation of the lime mud reburning kiln. Every time the clogged surface layer of the precoat has been removed by means of the scraper the filtering is more efficient and the lime mud volume is larger for a moment Further, also the dry solids content of the lime mud is higher after the precoat has been scraped which results in and calls for changes in the operation of the lime mud reburning kiln. When the whole precoat is renewed at the same time, the supply of the mud into the kiln is stopped altogether for a moment, which naturally causes a break in the operation of the kiln. Typically, the precoat is renewed once a shift, or correspondingly three times in 24 hours. In the long run, the wire is also so badly clogged that it must be washed either with acid or with a high-pressure washer. This operation also causes a long disturbance or interrupts the production of the lime mud reburning kiln.
The arrangement disclosed in U.S. Pat. No. 5,149,448 reduces the disadvantages and problems of the known techniques described above by removing a strip at a time from the filtering layer by means of a liquid jet reciprocating continuously in the longitudinal direction of the drum. This has been effected by installing one nozzle or several nozzles under the scraper, continuously reciprocating in the longitudinal direction of the drum. The high-pressure nozzles, for example two in number, have been provided under the scraper so that the purpose of one of the nozzles is to break the precoat layer and to drop it to the filter vat after which the second nozzle is used to wash the filter cloth clean. The pressure of the water sprayed from the washing nozzles is approx. 50-100 bar which gives a very good cleaning result. The nozzle disperses the jets so that the precoat is removed and the wire washed from a strip of approx. 10-50 mm in width at a time.
According to the arrangement of the above US patent, the washing nozzles have been installed in a rack designed for this purpose for moving the nozzles sideways. The nozzles are moved continuously to and fro in the longitudinal direction of the drum thus continuously renewing the precoat and cleaning the filter cloth. The advantages provided by this kind of continuous high-pressure water scraping are:
the dry-solids content of the lime mud from the lime mud filter to the lime mud reburning kiln is continuously uniform which allows smooth running of the kiln; PA1 change of the precoat is not needed as the precoat is renewed continuously a strip at a time; thus disturbances in the operation of the kiln due to a change of the precoat are avoided; PA1 it is not necessary to interrupt the filtering operation for a wash of the wire. PA1 the size of the needed apparatus is smaller than that of a drum filter having the same filtering capacity; PA1 maintaining the rated filtering capacity in severe process conditions due to a fast change of clogged precoat (the filtering surface of the sector is short, 0.6-1 m, whereby the jet cleans the surface in a short time compared to a drum filter, the length of the filtering surface of which is 5-12 m.) The result is that the disc filter can be rated for a higher surface load than a drum filter, which reduces the size of the filter; PA1 no massive, traversing mixer is needed in the filter vat; PA1 apparatuses having a high filtering capacity can easily be constructed; PA1 almost the whole of the filtering capacity range of drum filters can be covered with discs of a single size in diameter, which results in savings brought about by serial/modular construction in manufacturing, lighter blocks, thereby easier to handle, no massive large pools and no large drums demanding high construction accuracy (e.g. diameter 5 m, length 12 m); PA1 transport sizes are essentially reduced; PA1 the small size of the sectors enables a metal wire to be used; PA1 heavy equipment is not needed for moving the nozzles, because the maximal traversing distance of the nozzles is only about 1 meter, compared to distances of over 10 meters in drum filters; PA1 easier construction techniques than with drum filters, and PA1 exploitation of module technique in manufacture and rating for the mill. It is easy to rate the apparatus for different uses, as both the discs and the vats surrounding them are series-produced components, any number of which can be connected sequentially.
According to the invention, the lime mud filtering by means of a disc filter is effected almost correspondingly: The lime mud is pumped via a feed duct or ducts into a dividing duct, wherein the lime mud is kept in a homogenous state by mixing means. From the dividing duct the lime mud is evenly distributed to the trough-like vats of the filtering means, i.e. the disc filters. In the vats, the lime mud is retained homogenous by mixing means located on the periphery of the filter discs In other words, the lime mud is prevented from thickening on the bottom of the vat with the above-mentioned mixing means.
Most usually, the filter discs rotate in atmospheric pressure, whereby the pressure difference needed for the filtering is formed by means of a suitable vacuum source, such as drop leg, vacuum pump or a centrifugal pump, inside the discs formed by sectors. Nevertheless, it is also possible to pressurize the whole filter, whereby the discs would be surrounded by elevated pressure The partial vacuum, or more generally, the pressure difference causes a solid lime mud layer to begin to form on the wire-coated surfaces of the sectors. Scrapers have been arranged at a pre-determined distance from the surface of the wire on both sides of each of the filter discs so as to scrape the thickened lime mud layer from the precoat layer when the thickened layer has increased sufficiently in thickness. The lime mud to be removed falls onto a belt conveyor located below the filter discs, the conveyor then further transporting the lime mud to, e.g. a lime mud reburning kiln.
The filtrate removed from the lime mud by means of partial vacuum into the inside of the filter discs is directed via a flow channel to the hollow main shaft and further to a filtrate tank. From the filtrate tank the filtrate is further pumped into the process.
As is known, the surface layer of the precoat becomes clogged while the filtering proceeds, and must be removed from time to time. The technique used most usually is to move the scrapers closer to the filter surface, whereby they cut the clogged surface layer of the precoat off. This, nevertheless, causes trouble in the lime reburning kiln, as the amount of lime mud to be transported to the kiln increases suddenly, and, additionally, the dry solids content of the lime mud changes dramatically.
When the situation is such that the whole of the precoat layer is clogged, it must be removed completely. According to known technique, this is effected either by blowing air under the wire surface, i.e. inside the sector, or by washing the wire surface with water. These operations, like the partial removal of the precoat, cause problems in the operation of the lime reburning kiln. Namely, if the precoat is completely removed by blowing, the production of lime mud is interrupted temporarily, and the operation of the lime reburning kiln is disturbed. On the other hand, this necessitates the temporary feeding into the kiln of a larger, more dirty and damp portion of lime mud, which causes the end temperature of the kiln to decrease, which causes an increase in the energy consumption of the kiln.
If the precoat is removed by washing, the production is interrupted similarly, but because of its high water content the water-washed precoat cannot be transported to the lime reburning kiln, whereby the feed to the kiln is interrupted for a longer period than by removing the precoat by-blowing. Additionally, washing the precoat dilutes the thickness of the lime mud in the tank, which necessitates a separate, larger lime mud tank to eliminate the changes in thickness. In any case, the removal of precoat by washing with water decreases the capacity of the filter.
The international patent application PCT/FI94/00134, the disclosure of which is included herein in its entirety, deals with solutions to similar problems in drum filters. The above-mentioned publication explains in detail, how the precoat layer is needed especially in the filtering of lime mud, onto which the actual lime mud is thickened. Further, the PCT-application deals with various ways to prevent clogging of the precoat layer.
A way of cleaning the filtering drum by utilizing at least one jet of liquid so that at least one low-pressure jet of liquid moving longitudinally across the drum is used to loosen a strip of the thickened lime mud before the lime mud cake is removed from on top of the precoat by means of a scraper is disclosed as the invention in the above-mentioned patent application. Further, the jet is used to remove a portion of the precoat layer. According to said invention, a high-pressure jet of liquid is advantageously used to remove a strip of the whole thickness of the precoat layer while the surface of the strip of the filter drum is cleaned by means of another high-pressure jet of liquid before the lime mud cake is removed from on top of the precoat by means of a scraper.
The present invention relates to utilizing the technique disclosed in the above-mentioned publication in connection with a disc filter. As with drum filters, the jets in disc filters can also be located either under the scrapers or on the same side of the disc as the nozzles for washing the dried lime mud.
As advantages of using a disc filter for filtering lime mud the following factors, among others, can be mentioned:
The disc filter according to the invention can also be used for filtering green liquor. Because the filtering is advantageously effected by means of partial vacuum, the green liquor is cooled down, wherefore no separate green liquor cooler is needed prior to slaker-classifier. The fast method of changing the precoat also facilitates uninterrupted operation, which further optimizes the prerequisites for filtering the green liquor, a task usually very difficult to effect. A further advantage is also the small size of the equipment.
The above-mentioned drawbacks of the prior art equipment are eliminated and the above listed advantages are achieved by means of the method and apparatus according to the invention, the characterizing features of which are disclosed in the appended claims.